This invention relates in general to electrophotography, and more particularly, to a process for preparing carrier materials useful in the magnetic-brush type development of electrostatic latent images.
The formation and development of images on the surface of photoconductive materials by electrostatic means is well known. The basic electrostatographic process, as taught by C. F. Carlson in U.S. Pat. No. 2,297,691, involves placing a uniform electrostatic charge on a photoconductive insulating layer, exposing the layer to a light and shadow image to dissipate the charge on the areas of the layer exposed to the light and developing the resulting electrostatic latent image by depositing on the image a finely-divided electroscopic material referred to in the art as "toner". The toner will normally be attracted to those areas of the layer which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This powder image may then be transferred to a support surface such as paper. The transferred image may subsequently be permanently affixed to the support surface as by heat. Instead of latent image formation by uniformly charging the photoconductive layer and then exposing the layer to a light and shadow image, one may form the latent image by directly charging the layer in image configuration. The powder image may be fixed to the photoconductive layer if elimination of the powder image transfer step is desired. Other suitable fixing means such as solvent or overcoating treatment may be substituted for the foregoing heat fixing step.
Many methods are known for applying the electroscopic particles to the electrostatic latent image to be developed. One development method, as disclosed by E. N. Wise in U.S. Pat. No. 2,618,522 is known as "cascade" development. In this method, a developer material comprising relatively large carrier particles having finely-divided toner particles electrostatically clinging to the surface of the carrier particles is conveyed to and rolled or cascaded across the electrostatic latent image-bearing surface. The composition of the toner particles is so chosen as to have a triboelectric polarity opposite that of carrier particles. As the mixture cascades or rolls across the image-bearing surface, the toner particles are electrostatically deposited and secured to the charged portion of the latent image and are not deposited on the uncharged or background portions of the image. Most of the toner particles accidentally deposited in the background are removed by the rolling carrier, due apparently, to the greater electrostatic attraction between the toner and the carrier than between the toner and the discharged background. The carrier particles as unused toner particles are then recycled. This technique is extremely good for the development of line copy images. The cascade development process is the most widely used commercial electrostatographic development technique. A general purpose office copying machine incorporating this techique is described in U.S. Pat. No. 3,099,943.
Another technique for developing electrostatic latent images is the "magnetic brush" process as disclosed, for example, in U.S. Pat. No. 2,874,063. In this method, a developer material containing toner and magnetic carrier particles is carried by a magnet. The magnetic field of the magnet causes alignment of the magnetic carriers in a brush-like configuration. This "magnetic brush" is engaged with an electrostatic-image bearing surface and the toner particles are drawn from the brush to the electrostatic image by electrostatic attraction.
In magnetic-brush development of electrostatic latent images, the developer is commonly a triboelectric mixture of finely-divided toner powder comprised of dyed or pigmented thermoplastic resin mixed with coarser carrier particles of a soft magnetic material such as "ground chemical iron" (iron filings), reduced iron oxide particles or the like. The conductivity of the ferromagnetic carrier particles which form the "bristles" of a magnetic brush provides the effect of a development electrode having a very close spacing to the surface of the electrophotographic element being developed. By virtue of this development electrode effect, it is possible to develop part of the toners in pictures and solid blacks as well as line copy. Magnetic brush development sometimes makes this mode of developing advantageous where it is desired to copy materials other than simply line copy.
While ordinarily capable of producing good quality images, conventional developing materials suffer serious deficiencies in certain areas. Some developer materials, though possessing desirable properties such as proper triboelectric characteristics, are unsuitable because they tend to cake, bridge and agglomerate during handling and storage. Furthermore, with some polymer coated carrier materials flaking of the carrier surface will cause the carrier to have nonuniform triboelectric properties when the carrier core is composed of a material different from the surface coating thereon. In addition, the coatings of most carrier particles deteriorate rapidly when employed in continuous processes which require the recycling of carrier particles by bucket conveyors partially submerged in the developer supply such as disclosed in U.S. Pat. No. 3,099,943. Deterioration occurs when portions of or the entire coating separates from the carrier core. The separation may be in the form of chips, flakes or entire layers and is primarily caused by fragile, poorly adhering coating material which fails upon impact and abrasive contact with machines parts and other carrier particles. Carriers having coatings which tend to chip and otherwise separate from the carrier core or substrate must be frequently replaced thereby increasing expense and loss of productive time. Print deletion and poor print quality occur when carriers having damaged coatings are not replaced. Fines and grit formed from carrier disintegration tend to drift to and form undesirable and damaging deposits on critical machine parts.
Another factor affecting the stability of the triboelectric properties of carrier particles is the susceptibility of carrier coatings to "toner impaction". When carrier particles are employed in automatic machines and recycled through many cycles, the many collisions which occur between the carrier particles and other surfaces in the machine cause the toner particles carried on the surface to the carrier particles to be welded or otherwise forced onto the carrier surfaces. The gradual accumulation of impacted toner material on the surface of the carrier causes a change in the triboelectric value of the carrier and directly contributes to the degradation of copy quality by eventual destruction of the toner carrying capacity of the carrier. This problem is especially aggravated when the carrier particles, and particularly the carrier cores, are prepared from materials such as iron or steel having a high specific gravity or density since during mixing and the development process the toner particles are exposed to extremely high impact forces from contact with the carrier particles. It is apparent from the descriptions presented above as well as in other development techniques, that the toner is subjected to severe physical forces which tend to break down the particles into undesirable dust fines which become impacted onto carrier particles. Various attempts have been made to decrease the density of the carrier particles and reduce the concentration of the magnetic component by admixture of a lighter material, such as a resin, either in the form of a coating or as a uniform dispersion throughout the body of the carrier granule. This approach is useful in some instances but the amount of such lighter material sufficient to produce a substantial decrease in density has been indicated as seriously diminishing the magnetic response of the carrier particles as to cause a deterioration in the properties of a magnetic brush formed therefrom. One such attempt is disclosed in U.S. Ser. No. 699,030, filed Jan. 18, 1968, wherein the carrier particles comprise a low density, non-magnetic core such as a resin, glass, or the like having coated thereon a thin, continuous layer of a ferromagnetic material. It is therein indicated that a coating of finely powered iron or other subdivided ferromagnetic material does not show the high response to a magnetic field which is displayed by the continuous layers of the invention. Another earlier attempt at low density carrier materials is disclosed in U.S. Pat. No. 2,880,696 wherein the carrier material is composed of particles having a magnetic portion. The core therein may consist entirely of a magnetic material, or it may be formed of solid insulating beads such as glass or plastic having a magnetic coating thereon, or the core may consist of a hollow magnetic ball. However, for unknown reasons, the recited materials have apparently never been commercially successful. Thus, there is a continuing need for a better developer material for developing electrostatic latent images.